1. Field of the Invention
The present invention relates to method and apparatus for controlling an automatic transmission having a plurality of operating positions, to effect an automatic shifting operation from one position to another, and more particularly to improvements in such control method and apparatus, for selecting the optimum operating position of the transmission, with a relatively simple control program.
2. Discussion of the Prior Art
An automatically shifted transmission having a plurality of operating positions is widely used for motor vehicles, for example. The transmission includes a gear mechanism, a plurality of hydraulically operated frictional coupling units or devices associated with the gear mechanism, and a hydraulic actuator for operating the frictional coupling units. The frictional coupling units are automatically engaged or disengaged by the hydraulic actuator, in order to selectively establish the operating positions of the transmission.
A control apparatus for controlling the hydraulic actuator generally includes a shift lever operated by a vehicle driver, a shift position sensor for detecting the currently selected position of the shift lever, a vehicle speed sensor for detecting the running speed of the vehicle, and a throttle position sensor for detecting the opening of a throttle valve of the vehicle, which is used as a parameter that reflects or represents the output of the vehicle engine desired by the vehicle driver. In response to the signals from the shift position sensor, vehicle speed sensor and throttle position sensor, the control apparatus regulates the hydraulic actuator to control the engaging and disengaging actions of the frictional coupling units, for selecting the optimum operating position of the transmission, according to predetermined shift pattern relationships between the vehicle speed and the throttle opening, and depending upon the currently selected position of the transmission.
An example of the shift pattern relationships is illustrated in FIG. 22. Suppose the vehicle is running in a condition indicated by point "A" in FIG. 22, i.e., at a running speed n1, and with the throttle valve opening set at .theta.1 and the transmission placed in its 4th-speed position, and suppose the accelerator pedal is depressed to increase the throttle opening from .theta.1 to .theta.2 as indicated by point "B" in the figure, the automatic transmission is shifted from the 4th-speed position to the 3rd-speed position, since the transition from the point "A" to the point "B" results in the passage across a boundary (4.fwdarw.3) for shifting down the transmission from the 4th-speed position to the 3rd-speed position.
The conventionally controlled automatic transmission is shifted up and down, principally according to shift-up and shift-down boundaries, which are predetermined shift pattern relationships between the throttle opening and the vehicle running speed, as exemplified above. These shift pattern relationships or boundaries as illustrated in FIG. 22 are suitably modified or adjusted, depending upon the running condition of the vehicle. Various modes of adjustment of the shift pattern relationships have been proposed.
For instance, laid-open Publication No. 62-63251 of unexamined Japanese Patent Application discloses an arrangement wherein the signal generated by a vehicle speed sensor is compensated for by a temperature of the lubrication oil in the automatic transmission, so that shifting actions of the transmission occur at comparatively higher speeds of the vehicle with the same throttle opening when the oil temperature is relatively low, while the shifting actions occur at comparatively lower vehicle speeds with the same throttle opening when the oil temperature is relatively high. This arrangement permits to maintain the engine speed at a relatively high level while the engine is relatively cold, whereby the warming-up of the engine is accelerated, and the vehicle is smoothly driven.
Laid-open Publication No. 48-9729 of examined Japanese Patent Application proposes a technique for adjusting the shift pattern relationships depending upon the detected steering angle of the vehicle, so that shift-down actions of the transmission are inhibited to avoid a driver's unexpected increase in the drive force, when the steering angle exceeds a predetermined limit.
Laid-open Publication No. 62-37549 of unexamined Japanese Patent Application discloses a technique for adjusting the shift pattern relationships depending upon the detected gradient of the road surface on which the vehicle is running.
In the conventional methods of controlling the automatic transmission as indicated above, the shift pattern relationships between the throttle opening and the vehicle speed must be adjusted or compensated for, depending upon the selected running condition of the vehicle such as the transmission oil temperature, steering angle or road surface gradient, while satisfying the driver's requirement.
Described more specifically, the control apparatus for the automatic transmission must have many sets of shift pattern relationships which correspond to different values of the selected running parameter represented by a signal received from the appropriate sensor such as the temperature sensor. Alternatively, a set of basic shift pattern relationships for the standard or normal vehicle running condition must be adjusted by the signal received from the sensor. In the former case, the control apparatus must use a large-capacity memory for storing data representative of many sets of shift pattern relationships. In the latter case, the control apparatus tends to be complicated due to additional control programs executed to effect the adjustments of the basic shift pattern relationships.
Another problem encountered in the conventional control arrangements is derived from the use of the throttle opening and the vehicle running speed as the basic parameters for selecting the optimum operating position of the automatic transmission. That is, the control program does not directly deal with the optimum operating position of the transmission per se, but is executed to indirectly determine the optimum position of the transmission depending upon the adjusted throttle opening-vehicle speed relationships. As a result, the adjustment of the shift-pattern relationships depending upon two or more parameters (such as the oil temperature and road surface gradient) may sometimes cause a driver's unexpected shifting action of the transmission, i.e., the newly selected position of the transmission being contrary to the requirement or desire of the driver under a certain vehicle running condition.
As described above, while the transmission oil temperature is relatively low, a shift-up action of the transmission can occur only after the vehicle speed exceeds a relatively high level, that is, only after the engine speed exceeds a relatively high level. Similarly, the engine speed is maintained at a relatively high level to provide a comparatively increased drive force when the vehicle is running a relatively steep uphill road. Therefore, as a result of the adjustment of the shift pattern relationships by the oil temperature and road surface gradient, the transmission will not be shifted up until the engine speed rises to a considerably high level, where the transmission oil temperature is relatively low and the road surface gradient is relatively large.
Another drawback arises if the shift pattern relationships are adjusted by the steering and the road surface gradient. That is, the control apparatus cannot deal with two contradictory requirements, i.e., the inhibition of a shift-down action of the transmission to avoid an unexpected increase in the vehicle drive force while the steering angle is larger than the predetermined limit, on the one hand, and the requirement for a shift-down action to increase the drive force while the road surface gradient is larger than the predetermined limit, on the other hand. This situation may occur when the vehicle is running up a steep curved slope. This sort of interference between the different control parameters often occurs and cannot be ignored if the number of the control parameters is relatively large. However, it is hardly possible to eliminate such a control interference, for smooth running of the vehicle as desired by the driver, with the transmission placed in the optimum operating position.
Recently, there is a growing tendency toward increasing the number of control parameters or vehicle running conditions for controlling the automatic transmission in a more sophisticated way. However, the practically resulting outcome is away from the optimum control of the transmission that meets the driver's requirement. This tendency increases with the number of the control parameters.